Lateral stretching of the Philippine Sea Plate subducting along the Nankai‐Suruga Trough

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Abstract
Focal mechanisms are investigated for shallow earthquakes (h ≤60 km) of central Japan by using telemetered seismic data of microearthquakes. Shallow earthquakes in central and southwestern Japan can be distinguished into three groups, upper crustal earthquakes, subcrustal earthquakes, and interplate thrust earthquakes, the first and the second ones being subjects of the present study. Most of the upper crustal earthquakes are of strike slip type or reverse faulting with E‐W to SE‐NW compression, whereas the subcrustal earthquakes are characterized by strike slip or normal faulting with E‐W to NE‐SW extension. The subcrustal seismic zone, subparallel to the Nankai trough, becomes progressively deeper landward to the north and progressively shallower to the east so that it emerges into a very shallow seismic activity in the Izu Peninsula which has been interpreted as a microcontinent of the Philippine Sea plate colliding against the Eurasian plate. On the basis of hypocentral distribution, three‐dimensional seismic velocity structure, and focal mechanisms, subcrustal earthquakes are considered to occur within the subducting Philippine Sea plate that extends down to a depth of at least 60 km. On the basis of this and other focal mechanism studies, stress trajectories are drawn for the subducting Philippine Sea plate and for the overriding Eurasian plate. The overriding plate is, on the whole, under the E‐W compression which coincides with the moving direction of the Pacific plate relative to the Eurasian plate. The subducting plate, on the other hand, is under lateral extension along its strike. We interpret that the latter is a stress regime unique to an incipient subduction zone, where the subducting plate at subcrustal depths is forced to warp downward, being constrained laterally at the arc‐arc junctions, and thereby comes under lateral stretching. The lateral constraints arise either from apparent increase in flexural strength of the plate at the arc‐arc junction or from buoyancy of a microcontinent that resists subduction.